Recent aircraft designs have begun to employ an AC starter-generator system which may be used to start the main engines or auxiliary power unit (APU) of an aircraft when operating as a motor, and to supply electrical power to the aircraft electrical bus when operating as a generator. When operating as a motor, a starter-generator is therefore designed to supply mechanical output torque sufficient to start the engines.
The common construction of an aircraft starter-generator includes three separate brushless generators, namely, a permanent magnet generator (PMG), an exciter generator, and a main motor/generator. The PMG includes permanent magnets on its rotor. When the PMG rotor rotates, AC currents are induced in stator windings of the PMG. These AC currents are typically fed to a regulator or a control device, which in turn supplies a DC excitation current when the starter-generator is operating in a generator mode. Conversely, when the starter-generator is operating in a motor mode, the control device supplies AC excitation current.
If the starter-generator is operating in the generator mode, DC current from the regulator or control device is supplied to stator windings of the exciter. As the exciter rotor rotates, three phases of AC current are typically induced in the exciter rotor windings. Rectifier circuits that rotate with the exciter rotor rectify this three-phase AC current, and the resulting DC currents are provided to the rotor windings of the main motor/generator. Finally, as the main motor/generator rotor rotates, three phases of AC voltage are typically induced in the main motor/generator stator, and this three-phase AC power can then be provided to a load.
If the starter-generator is operating in the motor mode, AC power from the control device is supplied to the exciter stator. This AC power induces, via a transformer effect, an electromagnetic field in the exciter armature, whether the exciter rotor is stationary or rotating. The AC currents produced by this induced field are rectified and supplied to the main motor/generator rotor, which produces a DC field in the rotor. A power converter, which may be referred to as a start converter, supplies variable frequency AC power to the main motor/generator stator. This AC power produces a rotating magnetic field in the main stator, which causes the main rotor to rotate and thus supply mechanical output power to a starter-generator shaft. The bulk of the electrical power required to produce the engine starting torque is provided by the start converter. The size, weight, cost, and reliability of the start converter is primarily dependent upon the rated output current during the start mode.
Typically, starter-generators, such as the one described above, are designed to provide a standard rated voltage when operating in the generator mode and driven at a rated speed. In particular, the main motor/generator stator windings are wound with a number of turns that suitably supplies the standard rated voltage when operating in the generator mode. Although starter-generators designed in this manner provide satisfactory service, the machine voltage during the start mode is relatively low, and the system design is penalized. For example, when operating in the motor mode, main motor/generator stator windings wound with a conventional number of turns will generate a relatively low machine voltage. As a result, the electrical current needed to develop a desired level of starting torque can be relatively high. The relatively high current can cause undesirable electrical losses and heating of the starter-generator and of the start converter.
Hence, there is a need for a more efficient starter-generator system and method that can supply a standard rated voltage when operating in the generator mode, yet draws less current as compared to currently used systems, while developing a desired level of torque, when operating in the motor mode. The present invention addresses one or more of these needs.